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JAVA Pass by reference error in method

I was trying to perform sorting of integers in an array and it worked fine.
But when i try to modify the program by including a "pass by reference" concept via a method, it is throwing error "cannot find symbol".
I am new to JAVA and learning by my own, Please help me with what I am doing wrong here.
import java.util.*;
import java.io.*;
public class Sort {
public static void main(String[] args) {
Sort obj = new Sort();
Scanner in = new Scanner(System.in);
int i, p, k, arr[];
arr = new int[10];
System.out.println("Enter the numbers for sorting \n");
for (i = 0; i < 5; i++) {
arr[i] = in.nextInt();
}
for (i = 0; i < 5; i++) {
for (p = 0; p < 5; p++) {
if (arr[i] < arr[p]) {
/*
* moving the below block for swapping to a new method. k =
* arr[i]; arr[i]= arr[p]; arr[p]= k;
*/
obj.swap(obj);
}
}
}
System.out.println("\n");
for (i = 0; i < 5; i++)
System.out.println(arr[i]);
}
public void swap(Sort m) {
m.k = m.arr[i];
m.arr[i] = m.arr[p];
m.arr[p] = m.k;
}
}
The error I am getting is :
"Sort.java:44: error: cannot find symbol
m.k = m.arr[i];
^
"
Similarly 10 such errors for other variables as well.

You are trying to use index variables (i and p) that don't exist in the context you are trying to use them (inside swap() method body) as well as members of Sort (k and arr) which don't exist. The scope of all these, you have limited to the method body of main():-
public void swap(Sort m) {
m.k = m.arr[i]; //No 'i' in swap(). No 'k' or 'arr' in 'm'(an instance of 'Sort')
m.arr[i] = m.arr[p]; //No 'p' in swap()
m.arr[p] = m.k;
}
Short-term Solution
Change your swap() method to
//Now accepting in i and p
public void swap(Sort m, int i, int p) {
m.k = m.arr[i];
m.arr[i] = m.arr[p];
m.arr[p] = m.k;
}
then call it like this
obj.swap(obj, i, p); //pass in i and p
and move your Sort variables to be accessible members of Sort
public class Sort {
public static int k; //now accessible with m.k
public static int[] arr = new int[10]; //now accessible with m.arr
...
}
Lastly, is it intentional that your array is 10 long but you only fill it with 5 numbers?
Pass-by-Reference
There is no "pass-by-reference" in Java. Everything is passed by value. The confusing thing is that what is passed by value is technically a reference to the object, meaning you get strange effects like you can edit the object but not reassign it.

Solution: move the stuff back from the swap method to where it was.
Alternatively, provide the necessary values as parameters to swap.

Java - "the method ___ is undefined for the type ___"

I'm building a simple genetic algorithm based off this guide. I have made an 'Individual' class, which is as follows.
package simpleGA;
public class Individual {
public static int defaultGeneLength = 64;
private static byte genes[] = new byte[defaultGeneLength];
private int fitness = 0;
public void generateIndividual(){
for (int i = 0; i < defaultGeneLength; i++){
byte gene = (byte) Math.round(Math.random());
genes[i] = gene;
}
}
public byte getGene(int index) {
return genes[index];
}
public int size(){
return genes.length;
}
public int getFitness(){
if (fitness == 0){
fitness = FitnessCalc.getFitness(this);
}
return fitness;
}
}
I have also made a class called 'FitnessCalc', which is as follows.
package simpleGA;
import java.util.Scanner;
public class FitnessCalc {
static byte[] solution = new byte[64];
static String newSolution;
static Scanner input = new Scanner(System.in);
static void setSolution(){
do{
newSolution = null;
System.out.println("Please enter a string of '1's and '0's of length 64.");
newSolution = input.next();
}
while(newSolution.length() != 64);
solution = new byte[newSolution.length()];
for (int i = 0; i < newSolution.length(); i++) {
String character = newSolution.substring(i, i + 1);
solution[i] = Byte.parseByte(character);
}
}
static int getFitness(Individual individual){
int fitness = 0;
for (int i = 0; i < solution.length && i < individual.size(); i++) {
if (individual.getGene(i) == solution[i]) {
fitness++;
}
}
return fitness;
}
}
My problem is that in the getFitness method in the 'FitnessCalc' class, the compiler returns an error for both the size and getGene methods, saying
"The method size() is undefined for the type Individual" and
"The method getGene(int) is undefined for the type Individual".
I'm not sure what this means. I've looked at other questions like this but none of the scenarios seem to relate to mine. Can anyone help?
EDIT:
After cleaning and rebuilding the project (I am using Eclipse), the errors remained, and a new error in my remaining class, 'Population', has appeared.
package simpleGA;
public class Population {
Individual[] individuals;
public Population (int populationSize, boolean initialise){
individuals = new Individual[populationSize];
if (initialise){
for (int i = 0; i < size(); i++){
Individual newIndividual = new Individual();
newIndividual.generateIndividual();
saveIndividual(i, newIndividual);
}
}
}
public int size(){
return individuals.length;
}
public void saveIndividual(int index, Individual indiv){
individuals[index] = indiv;
}
}
The error is with the generateIndividual method being called in the Population method, and says the same as the other two:
"The method generateIndividual() is undefined for the type Individual".
The generateIndividual method is defined in the 'Individual' class.

To anyone wondering, I got rid of the issue by rewriting the code slightly. I moved the getFitness method over to the 'Individual' class, and that seemed to work. After this I made sure that the 'Individual' class only used methods from within its own class, so that classes weren't dependent on each other.

How would I utilize a value returned from a method and implement it the main method in java?

Can I pass the return value from a method into the main method then utilize that value in another method? That sounds confusing but let me try to explain it better with some code...
public static void main(String[] args){
ArrayList<GeometricObject> geoList = new ArrayList<GeometricObject>();
findPositionLargestObject(geoList);
System.out.println("BIGGEST OBJECT AT "+ maxIndex +" AREA =
"+geoList.get(maxIndex).getArea());
showObjects(geoList.get(maxIndex));
}
//METHOD RETRIEVING INT OF ARRAYLIST
private static int findPositionLargestObject(
ArrayList<GeometricObject> geoList) {
int maxIndex = 0;
for (int i = 1; i < geoList.size(); i++) {
// AREA OF I COMPARES MAX INDEX
if (geoList.get(i).getArea() > geoList.get(maxIndex).getArea()) {
maxIndex = i;
}
}
return maxIndex;
}
// METHOD FOR PRINTING SINGLE OBJECT OF ARRAYLIST
private static void showObjects(GeometricObject geometricObject) {
System.out.println(geometricObject.toString());
}
Lets say I even instantiate the index in the main method such as
int maxIndex = 0;
I want the first method called to return the value, assign that value to the variable maxIndex then utilize that value for the showObjects method. Thanks for any insight that can be given to a coding novice like myself. Is instantiating the variable in the main method no good? What is the logic behind the JAVAC execution here?? The curriculum covered in my course feels like this is an enormous hole that needs to be filled. Basically, How do I utilize a value returned from a method then implement into another method?

Variables are only containers for a value bound to its type. If a method is returning a type, you can place it's return value in a variable located in another block of code. To provide a very basic example for an easier understanding of how this can work:
private String getString(int number) {
if (number == 2) {
return "Not One";
}
return "One";
}
private void printValue(String number) {
if (number.equals("One")) {
System.out.println("i is 1");
} else {
System.out.println("i is not one");
}
}
public static void main(String[] args) {
int i = 1;
String testNum = getString(i);//returns "One"
printValue(testNum);//output: i is 1
}
With this example in mind,
int maxIndex = findPositionLargestObject(geoList);
showObjects(geoList.get(maxIndex));
is valid.

Unless I'm missing something, assign the result of your function call. I suggest you program to the List interface. Also, if using Java 7+ you could use the diamond operator <> like
List<GeometricObject> geoList = new ArrayList<>(); // <-- diamond operator
// ... populate your List.
int maxIndex = findPositionLargestObject(geoList);
and then yes you can use the variable maxIndex

you can obtain the return value in main method like this,
int maxIndex=findPositionLargestObject(geoList);
Code:
public static void main(String[] args){
ArrayList<GeometricObject> geoList = new ArrayList<GeometricObject>();
int maxIndex=findPositionLargestObject(geoList);
System.out.println("BIGGEST OBJECT AT "+ maxIndex +" AREA =
"+geoList.get(maxIndex).getArea());
showObjects(geoList.get(maxIndex));
}
//METHOD RETRIEVING INT OF ARRAYLIST
private static int findPositionLargestObject(
ArrayList<GeometricObject> geoList) {
int maxIndex = 0;
for (int i = 1; i < geoList.size(); i++) {
// AREA OF I COMPARES MAX INDEX
if (geoList.get(i).getArea() > geoList.get(maxIndex).getArea()) {
maxIndex = i;
}
}
return maxIndex;
}
// METHOD FOR PRINTING SINGLE OBJECT OF ARRAYLIST
private static void showObjects(GeometricObject geometricObject) {
System.out.println(geometricObject.toString());
}

Method to get the sum of an ArrayList

I am getting the totals of various String ArrayLists such as [1,3,4]...
by parsing them into integers and getting the total. This worked when I coded each individual one, but when I made a method by passing in the total int value and arraylist I always get a value of zero.
A method would save a lot of time.
public class Playing {
static ArrayList<String> list;
static int Vigor;
public static void main(String[] args) {
list = new ArrayList<String>();
vigoroustotal(list,Vigor);
public static void Listtotal(String par, int tt) {
for (String s : par) {
int i = Integer.parseInt(s);
tt += i;
}
}

Any changes you do to tt inside your method won't be visible anywhere else, because Java passes everything by value. Make the method return an int instead.

Your mistake her is passing the function tt and expecting it to be modified. Java doesn't modify parameters passed to functions. The corrected code would be this:
public static int ListTotal(List<String> par) {
int tt = 0;
for (String s : par) {
int i = Integer.parseInt(s);
tt += i;
}
return tt;
}
and would be used like this:
Vigor = ListTotal(list);

The changes that are made to the integer tt in the Listtotal() method will not be visible anywhere else but that method. You can make that method return an integer to solve that!
public static int Listtotal(ArrayList<String> par) {
int tt = 0;
for (String s : par) {
int i = Integer.parseInt(s);
tt += i;
}
return tt;
}
And then you need to change the main method:
public static void main(String[] args) {
list = new ArrayList<String>();
Vigor = Listtotal(list);
}

As, said, you need to return the total since java passes everything by-value, so the int tt you pass in won't hold a reference to the Vigor variable outside the method.
Therefore, when you pass in primitive types such as (int, char, boolean, byte etc.), anything you do to them inside a method won't be visible outside the method.
However, when you pass in a reference type (Objects such as ArrayList), it is still passed-by-value but that value is a copy of the reference to the Object outside the method. So, in the populateList method bellow, I can just call ArrayList.add() on the input because this input, even though it is passed-by-value, still points to the original Object that was put into this method.
public class Playing {
static ArrayList<String> list;
static int Vigor;
public static void main(String[] args) {
list = new ArrayList<String>();
populateList(list);
Vigor = getListTotal(list);
System.out.println("Total is:\t" + Vigor);
}
public static void populateList(ArrayList<String> list) {
String[] sampleData = { "4", "7", "2" };
for(int i = 0; i < sampleData.length; i++) {
list.add(sampleData[i]);
}
}
public static int getListTotal(ArrayList<String> list) {
int tt = 0;
for (String s : list) {
int i = Integer.parseInt(s);
tt += i;
}
return tt;
}
}

What you want is the... let's call it CountingList,
public class CountingList {
private List<Integer> integers = new ArrayList<Integer>();
private int sum;
public add(String s) {
int value = Integer.parseInt(s);
integers.add(value);
sum += value;
}
private void updateSum() {
sum = 0;
for (int i : integers) {
sum += i;
}
}
}
Obviously, you'll want to expose the functionality you need to use outside of the class, but this is (one) way of encapsulating the behavior you're needing.

Calling a method from a class array gives NullPointerException

I've been searching a lot for this problem and I can't find a solution. I'm trying to build a mini-game and I have a method for creating platforms. I have a class with every platform parameters, and I made a class array so i can have multiple platforms at the same time.
Problem: When I try to call the method for constructing the platform by sending the parameters I want, it gives me a NullPointerException. The method was working before, but with everything static and so i couldnt have multiple instances of that class, and now I removed the static fields from the platform class and it gives me the NullPointerException every time I call the method.
I copied the part of the code that gives me the error, the error goes the following way:
public static void main(String[] args) {
Game ex = new Game();
new Thread(ex).start();
}
In Game class:
public Load_Stage load = new Load_Stage();
public Game() {
-other variables initializatin-
Initialize_Items();
load.Stage_1(); // <--- problem this way
In Load_Stage class:
public class Load_Stage {
public Platforms plat = new Platforms();
public void Stage_1(){
Stage_Builder.Build_Platform(200, 500, 300, plat.platform1);
Stage_Builder.Build_Platform(100, 200, 100, plat.platform1);
}
}
And inside the Stage_Builder class:
public class Stage_Builder {
public static final int max_platforms = 10;
public static Platform_1[] p1 = new Platform_1[max_platforms];
public static boolean[] platform_on = new boolean[max_platforms];
public Stage_Builder() {
for (int c = 0; c < platform_on.length; c++) {
platform_on[c] = false;
}
}
public static void Build_Platform(int x, int y, int width, ImageIcon[] type) { // BUILDS A PLATFORM
for (int b = 0; b < max_platforms; b++) {
if (platform_on[b] == false) {
p1[b].Construct(x, y, width, type); // <-- NullPointerException here
platform_on[b] = true;
break;
}
}
}
}
Thanks beforehand.
EDIT: Here's the Platform_1 class (sorry for forgetting about it):
public class Platform_1 {
private int platform_begin_width = 30;
private int platform_middle_width = 20;
public int blocks_number = 0;
public ImageIcon[] platform_floors = new ImageIcon[500];
private int current_width = 0;
public int [] platform_x = new int [500];
public int platform_y = 0;
public int platform_width = 0;
public void Construct(int x, int y, int width, ImageIcon [] type) {
platform_width = width;
platform_y = y;
for (int c = 0; current_width <= platform_width; c++) {
if (c == 0) {
platform_x[c] = x;
platform_floors[c] = type[0];
current_width += platform_begin_width;
} else if ((current_width + platform_middle_width) > platform_width) {
platform_floors[c] = type[2];
blocks_number = c + 1;
platform_x[c] = current_width + x;
current_width += platform_middle_width;
} else {
platform_floors[c] = type[1];
platform_x[c] = current_width + x;
current_width += platform_middle_width;
}
}
}
}
And the Platforms class:
public class Platforms {
public ImageIcon[] platform1 = {new ImageIcon("Resources/Sprites/Stage_Objects/Platform1/begin.png"),
new ImageIcon("Resources/Sprites/Stage_Objects/Platform1/middle.png"),
new ImageIcon("Resources/Sprites/Stage_Objects/Platform1/end.png")};
}

The problem and solution are both obvious.
public static Platform_1[] p1 = new Platform_1[max_platforms];
After this line of code executes, p1 is an array of references of type Platform_1 that are all null.
Executing this line of code tells you so right away:
p1[b].Construct(x, y, width, type); // <-- NullPointerException here
The solution is to intialize the p1 array to point to non-null instances of Platform_1.
Something like this would work:
for (int i = 0; < p1.length; ++i) {
p1[i] = new Platform1();
}

I'm not seeing where you put things in the p1 array in the Stage_Builder class.
Another possibility (unlikely, but possible if you haven't shown everything) is that something in the Platform class, which you didn't show, is not initialized, and is breaking when you call Construct.
Also, the following seems problematic
public static Platform_1[] p1 = new Platform_1[max_platforms];
public static boolean[] platform_on = new boolean[max_platforms];
public Stage_Builder() {
for (int c = 0; c < platform_on.length; c++) {
platform_on[c] = false;
}
}
it appears you declare static variables p1 and platform_on, but you only populate platform_on in a constructor. So the first time you create a Stage_Builder instance, you populate one static array with all false, and don't put anything in the other static array...
Populate those static variables in a static block
// static var declarations
static {
// populate static arrays here.
}

The array you are calling a message on was never filled.
You have
public static Platform_1[] p1 = new Platform_1[max_platforms];
so p1 is
p1[0] = null
p1[1] = null
.
.
.
p1[max_platforms] = null
You try to call
p1[b].Construct(x, y, width, type);
which is
null.Construct(...);
You need to initialize that index on the array first.
p1[b] = new Platform_1();
p1[b].Construct(...);

First of all, your problem is that p1[b] is most likely null, as duffymo pointed out.
Secondly, you are using arrays in a really weird way. What about
a) delete Stage_Builder
b) Instead, have an ArrayList somewhere
c) The equivalent of Build_Platform1() look like this, then:
p1.add(new Platform1(x, y, width, type);
d) no if on[i], no max_platforms, no for loop to add a platform (the latter is a bad performance problem if you actually have a few hundret platforms)